1. Field of the Invention
The present invention relates to an optical transmitter implemented with a semiconductor laser diode and a method to control the optical transmitter.
2. Related Background Art
Various prior arts have reported methods to prevent a semiconductor laser diode (hereafter denoted as LD) from emitting extraordinarily when a driving signal is initially provided to the LD. A Japanese Patent Application published as JP-H07-273388A has disclosed one of such methods, in which the optical transmitter provides a feedback loop that controls the bias current supplied to the LD. This feedback loop stops the supply of the bias current to the LD when the driving signal is lost. Another Japanese Patent application published as JP-2000-049715A has disclosed an optical transmitter that provides, even when the input driving signal is lost, a pseudo random pattern to the LD to operate the feedback loop to maintain the optical output power from the LD constant. An optical switch may suspend the light derived from the pseudo random pattern from outputting from the transmitter.
The method that stops the supply of the current to the LD when the input driving signal is lost stops the emission of the LD itself. The suspension of the light emission may cause a failure in equipments connected in downstream of the transmitter. For instance, no input optical signal may put in an out of order of the automatic gain control of the optical amplifier. Another method to supply the pseudo random signal to the LD is necessary to provide at least another two components, one of which is a signal generator of the pseudo random pattern and the other is an optical switch to prevent the light with the random pattern from being output from the transmitter in addition to the possibility to cause the failure in the downstream equipments.